Hey just got this app name AdvLock from cydia,which is a great security app.it allows you to password protect Apps from cydia,installer and also the app store.
in my few days of using this app i realize it was way better than the parental control lock for the iphone,it allows you to lock every app installed on your iphone.for example you can put a lock for your sms only,just so nobody could view your private messages.that`s just a few benefits of using this app,please visit the website

here is instruction on how to install mAdvLock v1.0 on iPhoneOS 2.0-2.2:
1) Jailbroken iPhone (3G iPhone)
2) Launch Cydia, and Choose the Utilities Category
3) Choose mAdvLock and install
if you have any problems or questions contact:madvlock@mcleaner.com